Pressure vessels are used for a large number of industrial applications. Pressure vessel walls may be constructed of a single layer or of multiple layers. Often a pressure vessel will contain one or more hemispherical heads because of the structural strength of a hemispherical shape. Such multi-layer pressure vessels are disclosed in U.S. Pat. Nos. 3,774,296, 3,785,040, 3,604,587 and 3,432,820. In single wall vessels, a hemispherical shape may be cast or milled as a single unit. In multi-layer shells, hemispherical heads are sometimes formed around a hemispherical inner layer by welding on successive layers of gores onto the inner layer in the fashion disclosed in U.S. Pat. Nos. 3,374,935 and 3,423,820. Typically, the main support during the manufacture of the hemispherical head is a hemispherical frame. Such frames have been formed by cutting arc segments from plate metal and welding a hemispherical frame from such arcs onto a base plate. Because of the imprecise nature of cutting, the hemispherical frame formed is not perfect and the construction of the head is made more difficult by the need to correct for such deficiencies during construction.
In addition, the depth of each arc segment required to provide the necessary strength for the frame produces a heavy frame which is difficult to dismantle. If, as is sometimes necessary, the frame must remain in place until the whole vessel is built, the frame must be cut by torch into small enough pieces to be withdrawn through an access hatch.
Because pressure vessels are often one-of-a-kind constructions, more expensive techniques of forming more perfect hemispherical frames by casting or machining are not economical. Therefore, with the prior art, the vessel manufacturer was faced with the choice of manufacturing a more precise and expensive frame or the more economical but less accurate frame for making the head.